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Hi, assuming that the luggage cover is in place and the roof is unlocked at the front the most likely cause of this problem is either loss of hydraulic fluid in the pump or a faulty pump or relay. You should first check the fluid level. Click here for instructions to check the fluid level. If the fluid level is okay the next step should be to scan the top control module for fault codes and to check to see if the pump is operational.
Let me know if you have any questions.
The control unit is next to the pump. You need a Mercedes scan tool to read the codes. Does the pump run when you jump terminal 30 to 87 in the relay connector using a fused jumper lead?
You have to close the luggage cover switch in order for the pump to operate.
Okay, sometimes the roof will synchronize itself if you manually lower the roof then raise it with the switch again. First try opening it and see if it will open. If the pump is actually turning off midway I'd suspect a bad pump or relay or possibly a fault with the wiring to the luggage cover switch.